DC Generators Questions and Answers

Answer: C

A transformer and a solid-state rectifier have no commutator to maintain and are cheaper for the same output, so dc is now almost always made from ac. The dc machine survives in traction and industrial installations that are still in service.

Answer: D

The emf equation, the magnetisation curve, armature reaction and commutation all reappear in the dc motor and in the drives built around it. It is also the clearest possible illustration of self-excitation and its conditions.

Answer: D

The losses in a DC machine are grouped as copper (electrical) losses, iron (core) losses, mechanical losses and brush losses. Hysteresis loss is the energy spent in turning the magnetic domains of the iron core round with each reversal of the flux seen by the armature core, and together with eddy current loss it forms the iron loss, also called the core loss.

Friction and windage are the mechanical losses, the \(I^2R\) loss in the windings is electrical, and brush losses come from the contact drop and brush friction. The answer is iron, option D.

Answer: A

The yoke of a DC machine is the outer frame that carries the field poles and the magnetic flux. In small machines, where cost matters more than weight, it is made of cast iron, which is cheap and easy to cast into shape. In large machines, where weight matters, cast steel or rolled steel is used instead because of its better magnetic properties.

Aluminium, copper and German silver are not magnetic materials, so they cannot carry the main flux of a machine. The answer is cast iron, option A.

Answer: A

The average pitch is (Y_B + Y_F)/2. In the armature winding of a DC machine, the back pitch Y_B and the front pitch Y_F are the distances traversed on the back and front of the armature, and the average of the two is called the average pitch.

\[ Y_A = \frac{Y_B + Y_F}{2} \]

The expression (Y_B - Y_F) is the resultant pitch of a lap winding, and half of that difference is not an average pitch, while the plain sum is not an average. So the first option is correct.

Answer: C

A 4-pole machine with a duplex lap winding has 8 parallel paths between the brushes. In a lap winding the number of parallel paths is equal to the number of poles multiplied by the multiplicity of the winding.

\[ A = mP = 2 \times 4 = 8 \]

A simple lap winding would give 4 paths and a duplex wave winding would give 4 paths too (2 times 2). The value 2 is for a simple wave winding and 16 is not the value for this winding. So the third option is correct.

Answer: D

The eddy current loss varies as \(t^{2}\).

The eddy current loss in a laminated core is

\[ P_{e} = K_{e} B_{m}^{2} f^{2} t^{2} V \]

where \(B_{m}\) is the peak flux density, \(f\) the frequency, \(t\) the lamination thickness and \(V\) the core volume. Halving the thickness cuts the loss to a quarter, which is why armature and transformer cores are built from thin insulated laminations.

The loss grows with thickness, so \(1/t\) and \(1/t^{2}\) have the relationship upside down, and a linear dependence on \(t\) understates it.

Answer: B

The commutator acts as a full wave rectifier.

The emf induced in each armature coil of a dc generator is alternating, reversing every time the coil passes from one pole to the next. The commutator reverses the coil's connection to the brushes at exactly that moment, so both half cycles reach the external circuit in the same direction. It is a mechanical full wave rectifier.

A half wave rectifier would pass only one half cycle and block the other, which the commutator does not do. 'Controlled' rectifiers, such as thyristor bridges, vary the point in the cycle at which conduction begins; the commutator's switching is fixed by the brush position, not controlled.